The present invention relates to brake-boosters especially for motor vehicles and applies more especially to adjusting the jump of such boosters.
A brake-booster as defined above generally comprises a control rod which moves towards the front of the vehicle when its driver operates the brake pedal. Valve 34 has a planar front surface 34A engaging valve seat 28A in FIG. 2. This displacement of the control rod is transmitted to an element called a plunger which in its turn operates boosting means. In general, these boosting means consist of a three-way valve whose function is to interrupt the communication between the forward and rear chambers of the booster, in which the same reduced pressure exists in the rest position, and to connect the rear chamber with a pressure source under a higher pressure in order to create a difference in pressure across the two faces of a piston separating these two chambers. Therefore the piston moves towards the front, solidly fixed to an output rod or push rod acting in its turn on a master cylinder, therefore generating an increase in pressure in the hydraulic brake circuit of the vehicle in order to brake the latter.
In a known manner, the piston serving to transmit the boosting force to the output rod or push rod acts on the latter via a reaction disk made from a deformable material such as an elastomer. In the rest position, there is a little play between the front end of the plunger and the reaction disk. The rear end of the plunger forms an annular valve seat coaxial with another annular valve seat formed on the piston, a member forming a valve co-operating with these valve seats.
In such boosters, the reaction to the brake pedal only becomes noticeable when the boosting force generated by the booster passes a certain threshold. This threshold is called the "jump" of the booster. It constitutes an important feature of the booster. In effect, while the existence of the jump is essential for an immediate braking response during the operation of the pedal, the vehicle manufacturers generally wish the value of this jump to remain within certain limits in order that the boosting does not attain too large a value without increasing the reaction to the brake pedal.
From DE-A-3 205 268 is known means for adjusting the load of the operating rod spring in order to control the jump value. Nevertheless, taking into account especially the manufacturing tolerances of the various pieces constituting the booster, large variations in the value of the jump may exist from one booster to another in a production run.
A method is known from document FR-A-2,480,898 for adjusting the value of the jump of a booster, a method according to which the play existing between the front face of the plunger and the reaction disk is adjusted. This method presents numerous disadvantages in that it is long and complicated and that in addition in order to carry it out it is necessary to use ultrasonic or heating means in order to deform plastically a valve part comprising a supplementary piece between the plunger and the reaction disk. In addition the major disadvantage of this method resides in the fact that the adjustment must be carried out before assembling the reaction disk, and consequently the booster itself. It follows that the various pieces have to be carefully matched.